High densities of information storage in disk drives require closely spaced concentric tracks on the disks in which the information is written. This requires precise positioning of the magnetic heads on the centers of the data tracks during a track following operation. Any magnetic head offset from track center degrades performance in reading or writing which is unacceptable. Disk drives conventionally comprise a stack of memory disks mounted in axially spaced positions on a common spindle to be rotated at a constant speeds. Corresponding tracks on the disk surfaces are cylindrically aligned. The magnetic heads are mounted at the ends of flexures on arms on a movable carriage. There is one magnetic head for each disk surface. These magnetic heads are radially and circumferentially aligned to be moved as a group to position a selected head at a selected track for reading or writing at that track location. In track seeking operations, the movable carriage moves the magnetic heads in a direction radially of the tracks. In a track following operation, the movable carriage maintains a magnetic head in a position centered with respect to a selected track. Tracks selected for reading or writing may have track numbers which when detected by the magnetic head are fed back to the controls for the movable carriage. In other instances, information is provided to the controls as to the number of track crossings required to move the magnetic head from a present track to a selected track.
Servo code is used to define the centers of tracks on the disks. Servo code is magnetically recorded in the form of magnetic dibits. The servo codes conventionally comprise either two phase or four phase magnetic dibit recordings which are radially aligned in a column for each phase in spaced end-to-end relation. The columns are in circumferentially spaced positions with the dibits in a column radially displaced with respect to the dibits in adjacent columns defining tracks. The difference in signals generated in the magnetic head scanning these magnetic dibits is used to develop power for the servo moving the movable carriage to center the magnetic head so that the signals become equal. Any amplitude distortion of these signals, by influences other than an off center position of the magnetic head with respect to magnetic dibits defining a specific track, results in the servo positioning the magnetic head in a radially offset position with respect to a track center, resulting in a condition called "track crowding", degrading track reading or writing performance.
High density packing of the magnetic dibits results in interference among the magnetic fields of the dibits. This is commonly called inter-symbol interference. This intersymbol interference is not uniform. The magnetic field at some dibits may be diminished and others may be increased in intensity. This variation in magnetic field intensity among the dibits will cause the servo system to position the magnetic heads away from true track center. This movement away from track center is not unidirectional and, hence, causes some tracks to appear more closely spaced and causes other tracks to appear further spaced. This can be referred to as track position syncopation. Offset of a magnetic head with respect to the center of a selected track degrades disk drive performance.